Spain's Paula Badosa roared past Belarus's top-seeded Aryna Sabalenka 6-4, 6-0 on Thursday in round-robin action at the season-ending WTA Finals.
Badosa, who has climbed into the top 10 after starting the season ranked 70th in the world, looked every inch the player who lifted the trophy at Indian Wells last month — when world number two Sabalenka was sidelined with Covid-19.
Although she had played only two matches since the US Open, Sabalenka grabbed an early 4-2 lead.
But she couldn't maintain the momentum in the challenging high-altitude conditions, and Badosa won 10 games in a row to claim the upset.
"I think I played pretty good," Badosa said. "The conditions are tough here to play, but I think I played an amazing match. She's an amazing player. I knew I had to play like this. I'm really happy with my match."
The Spaniard next faces Greece's Maria Sakkari, who beat former French Open champion Iga Swiatek 6-2, 6-4, while Sabalenka will face Swiatek as round-robin play continues.
The eight singles players in the elite season-ending event are split into two groups of four for round-robin play, with the top two finishers in each group advancing to the semi-finals.
The tournament was moved to Guadalajara from its usual home in Shenzhen, China, due to the coronavirus pandemic.
World number one Ashleigh Barty of Australia is not defending her title because of continuing pandemic travel restrictions.
Sakkari, the first Greek to qualify for the WTA Finals in either singles or doubles, needed just 85 minutes to subdue Swiatek.
She has beaten the Pole in all three of their career encounters, including at the quarter-finals at Roland Garros where Swiatek was the defending champion.
"I think it was a very solid match from my side," Sakkari said. "Obviously my serve really helped my game. I felt quite good with the altitude. I could control my shots pretty well. I think every day I'll feel even better.
"All three times I played her, I played one of the best matches of the season. Like even today, I think I was very solid in these conditions."